1. Field of the Invention
The present invention relates to an exposure apparatus designed to transfer a pattern formed on a mask or reticle onto a photosensitive substrate and used in a photolithographic process for manufacturing a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like and, more particularly, to a method and apparatus for positioning a photosensitive substrate with respect to a predetermined reference plane (e.g., the imaging plane of a projection optical system).
2. Related Background Art
Conventionally, an exposure apparatus incorporates a plane position detection unit for performing proximity gap setting, focusing, leveling, and the like. Especially in a projection exposure apparatus, when a reticle pattern is to be projected/exposed on a photosensitive substrate (a wafer or glass plate on which a photoresist is coated) via a projection optical system having a high resolving power, a surface of the photosensitive substrate must be accurately aligned with the imaging plane (the projection imaging plane for the reticle pattern) of the projection optical system, that is, focusing must be performed, as disclosed in U.S. Pat. No. 4,650,983.
In order to achieve proper focusing throughout the projection field of view of the projection optical system, some consideration needs to be given to the inclination of a partial area, on the projection optical system, which enters the projection field of view, i.e., one projection/exposure area (shot area). As a technique of performing a focusing operation in consideration of the inclination of the surface of one shot area on a photosensitive substrate, a technique disclosed in U.S. Pat. No. 4,558,949 and the like is known. Especially in U.S. Pat. No. 4,383,757, there is disclosed a technique of projecting the spots of light beams on four points on a photosensitive substrate via a projection optical system, and photoelectrically detecting spot images formed by the reflected light beams, thus performing focusing and inclination correction (leveling) with respect to the photosensitive substrate.
A multi-point oblique incident type focus detection system like the one disclosed in, e.g., U.S. Pat. No. 5,118,957 is also known as a system developed from the oblique incident type focus detection system disclosed in U.S. Pat. No. 4,558,949. In this system, pin hole images are projected on a plurality of points (e.g., five points) in a shot area on a projection optical system by an oblique incident scheme without the mediacy of a projection optical system, and the respective reflected images are received by a two-dimensional position detection element (CCD) at once. The system is generally called an oblique incident type multi-point AF system, which can execute focus detection and inclination detection with high precision.
As a conventional projection exposure apparatus, a reduction projection exposure apparatus of a step-and-repeat scheme, a so-called stepper, is widely used. This apparatus is designed to sequentially move shot areas on a photosensitive substrate into the projection field of view (exposure field) of a projection optical system to position them and expose a reticle pattern image on each shot area.
FIG. 27 shows the main part of a conventional stepper. Referring to FIG. 27, a pattern image on a reticle 51 is projected/exposed on each shot area on a wafer 53, on which a photoresist is coated, via a projection optical system 52 with exposure light EL from an illumination optical system (not shown). The wafer 53 is held on a Z leveling stage 54. The Z leveling stage 54 is mounted on a wafer-side X-Y stage 55. The wafer-side X-Y stage 55 performs positioning of the wafer 53 within a plane (X-Y plane) perpendicular to an optical axis AX1 of the projection optical system 52. The Z leveling stage 54 sets the focus position (the position in a direction parallel to the optical axis AX1) of an exposure surface (e.g., an upper surface) of the wafer 53 and the inclination angle of the exposure surface in designated states.
A movable mirror 56 is fixed on the Z leveling stage 54. A laser beam from an external laser interferometer 57 is reflected by the movable mirror 56 so that the X- and Y-coordinates of the wafer-side X-Y stage 55 are constantly detected by the laser interferometer 57. These X- and Y-coordinates are supplied to a main control system 58. The main control system 58 controls the operations of the wafer-side X-Y stage 55 and the Z leveling stage 54 through a driving unit 59 so as to sequentially expose pattern images of the reticle 51 on the respective shot areas on the wafer 53 by the step-and-repeat scheme.
In this case, the pattern formation surface (reticle surface) on the reticle 51 and the exposure surface of the wafer 53 need to be conjugate to each other with respect to the projection optical system 52. However, the reticle surface does not vary much because of the high projection magnification and the large depth of focus. In general, therefore, an oblique incident type multi-point AF system is used to only detect whether the exposure surface of the wafer 53 coincides with the imaging plane of the projection optical system 52 within the range of the depth of focus (i.e., whether an in-focus state is achieved), thus controlling the focus position and inclination angle of the exposure surface of the wafer 53.
In the conventional multi-point AF system, illumination light with which the photoresist on the wafer 53 is not sensitized, unlike the exposure light EL, is guided from an illumination light source (not shown) via an optical fiber bundle 60. The illumination light emerging from the optical fiber bundle 60 is radiated on a pattern formation plate 62 via a condenser lens 61. The illumination light transmitted through the pattern formation plate 62 is projected on the exposure surface of the wafer 53 via a radiation objective lens 65. As a result, a pattern image on the pattern formation plate 62 is projected/formed on the exposure surface of the wafer 53 obliquely with respect to the optical axis AX1. The illumination light reflected by the wafer 53 is re-projected on the light-receiving surface of a light-receiving unit 69 via a focusing objective lens 66, a vibration plate 67, and an imaging lens 68. As a result, the pattern image on the pattern formation plate 62 is formed again on the light-receiving surface of the light-receiving unit 69. In this case, the main control system 58 vibrates the vibration plate 67 through a vibrating unit 70, and detection signals from a large number of light-receiving elements of the light-receiving unit 69 are supplied to a signal processing unit 71. The signal processing unit 71 supplies, to the main control system 58, a large number of focus signals obtained by performing synchronous detection of the detection signals in response to a driving signal from the vibrating unit 70.
FIG. 28B shows opening patterns formed on the pattern formation plate 62. As shown in FIG. 28B, nine slit-like opening patterns 72-1 to 72-9 are arranged on the pattern formation plate 62 in a crisscross form. Since these opening patterns 72-1 to 72-9 are radiated on the exposure surface of the wafer 53 from a direction crossing the X- and Y-axes at 45.degree., projection images AF1 to AF9 of the opening patterns 71-1 to 72-9 are arranged in the exposure field, of the projection optical system 52, formed on the exposure surface of the wafer 53 in the manner shown in FIG. 28A. Referring to FIG. 28A, a maximum exposure field 74 is formed to be inscribed to the circular illumination field of view of the projection optical system 52, and the projection images of the slit-like opening patterns are respectively projected on measurement points AF1 to AF9 on the central portion and the two diagonal lines in the maximum exposure field 74.
FIG. 28C shows a state of the light-receiving surface of the light-receiving unit 69. As shown in FIG. 28C, nine light-receiving elements 75-1 to 75-9 are arranged on the light-receiving surface of the light-receiving unit 69 in a crisscross form, and a light-shielding plate (not shown) having slit-like openings is arranged above the light-receiving elements 75-1 to 75-9. Images of the measurement points AF1 to AF9 in FIG. 28A are respectively formed again on the light-receiving elements 75-1 to 75-9 of the light-receiving unit 69. In this case, the illumination light reflected by the exposure surface of the wafer 53 in FIG. 27 is reflected by the vibration plate 67, which is present at the pupil position of the focusing objective lens 66 and also vibrates (rotates/vibrates) about an axis substantially perpendicular to the drawing surface of FIG. 27. For this reason, as shown in FIG. 28C, on the light-receiving unit 69, the positions of the projection images formed again on the light-receiving elements 75-1 to 75-9 vibrate in a direction RD as the widthwise direction of each slit-like opening.
In addition, since the images of the slit-like openings on the respective measurement points AF1 to AF9 are projected obliquely with respect to the optical axis of the projection optical system 52, when the focus position of the exposure surface of the wafer 53 changes, the re-formation position of the projection images on the light-receiving unit 69 changes in the direction RD. Therefore, by performing synchronous detection of the respective detection signals from the light-receiving elements 75-1 to 75-9 in response to the vibration signal from a vibration plate 67 in the signal processing unit 71, nine focus signals corresponding to the focus positions of the measurement points AF1 to AF9 can be obtained. The inclination angle and focus position of the exposure surface are obtained from these nine focus positions and are supplied to the main control system 58. The main control system 58 sets the focus position and inclination angle of the shot area on the wafer 53 to predetermined values through the driving unit 59 and the Z leveling stage 54. In this manner, in the stepper, each pattern image of the reticle 51 is exposed while the focus position and inclination angle of each shot area on the wafer 53 are aligned with the imaging plane of the projection optical system 52.
As described above, in the stepper, after each shot area on a wafer is positioned in the exposure field of the projection optical system, the focus position and inclination angle of the exposure surface of each shot area are detected by using the multi-point AF system, thus setting the entire exposure surface in the depth of focus of the projection optical system. For this reason, a long processing time is required for each shot area, resulting in a low throughput. As disclosed in U.S. Pat. No. 4,874,954, there is a method of eliminating such an inconvenience. In this method, while an X-Y stage is moved, focus positions are detected at predetermined points in a shot area which is to be exposed next on a wafer, and a Z leveling stage is finely moved to perform focusing with respect to the shot area. In the method, however, if a stepped portion is present in a shot area, it is difficult to perform accurate focusing with respect to the exposure surface (average plane) of the shot area. In addition, leveling of the shot area cannot be performed, and hence the entire surface cannot be set within the depth of focus of a projection optical system.
With a recent trend toward larger semiconductor elements, an increase in area of a pattern which can be transferred onto a wafer by one projection/exposure operation is required. Consequently, the field size of a projection optical system tends to increase. In addition, with a reduction in pattern size of a semiconductor element, a projection optical system is required to have a higher resolving power. It is, however, very difficult to realize both a broad field and a high resolving power. If, for example, an attempt is made to increase the resolving power while ensuring a field size equivalent to that in the prior art, the imaging performance (associated with distortion, curvature of field, and the like) cannot be maintained throughout the exposure field. Under the circumstances, in order to properly respond to the tendencies toward larger areas of transfer patterns and finer transfer patterns, a scan projection exposure apparatus has been reconsidered. This apparatus is designed to simultaneously scan a reticle and a wafer with respect to a projection optical system when a reticle pattern is projected/exposed on the wafer.
As a conventional scan exposure apparatus, an apparatus having a one-to-one magnification type reflecting projection optical system is known. In this apparatus, a reticle stage for holding a reticle and a wafer stage for holding a wafer are coupled to a common movable column and are scanned/exposed at the same speed. Since this one-to-one magnification type reflecting projection optical system uses no refracting element (lens), it exhibits a good chromatic aberration property throughout a wide exposure light wavelength range. The optical system simultaneously uses two or more line spectra (e.g., g- and h-rays) from a light source (mercury lamp) to increase the intensity of exposure light so as to allow a scan/exposure operation at a high speed. In the reflecting projection system, however, a point at which astigmatism values caused by both an S (sagittal) image plane and an M (meridional) image plane are made zero is limited to a position near an image height position separated from the optical axis of the reflecting projection system by a predetermined distance. For this reason, exposure light illuminating a reticle is shaped like a part of a narrow ring, a so-called arcuated slit.
As still another conventional scan exposure apparatus, an apparatus incorporating a refracting element is also known. In this apparatus, while the projecting magnification is increased or decreased by the reflecting element, both a reticle stage and a wafer stage are relatively scanned at a speed ratio corresponding to the projecting magnification. In this case, as a projection optical system, a system constituted by a combination of a reflecting element and a refracting element or a system constituted by only a refracting element is used. As an example of the reduction projection optical system constituted by a combination of a reflecting element and a refracting element, the system disclosed in U.S. Pat. No. 4,747,678 is available. U.S. Pat. No. 4,924,257 also discloses a method of performing step-and-scan exposure by using a reduction projection optical system capable of full field projection. In such a projection optical system incorporating a refracting element, exposure light illuminating a reticle has a rectangular or hexagonal shape.
In the scan exposure apparatus, similar to the stepper, exposure needs to be performed while an exposure surface of a wafer is aligned with the imaging plane of the projection optical system. For this reason, focusing and leveling may be performed by using the conventional multi-point AF system (FIG. 27) used by the stepper without any modification. In the conventional multi-point AF system, however, since measurement points are set in the exposure field of the projection optical system, focusing of a wafer may be made inaccurate owing to, e.g., the influence of a phase delay based on a signal processing time taken in the multi-point AF system. More specifically, in the scan exposure apparatus, a wafer is scanned with respect to the exposure field of the projection optical system. Even if, therefore, the wafer is finely moved along the optical axis of the projection optical system on the basis of focus positions detected at the respective measurement points in the exposure field, the wafer has already been moved by a predetermined distance at this time, and focusing cannot be always performed accurately. In order to prevent this, the moving speed of the wafer stage during a scan/exposure operation may be decreased. In this method, however, the exposure time required for each shot area is prolonged to cause a great reduction in throughput. In addition, in a leveling operation, similar to a focusing operation, leveling of the wafer is made inaccurate owing to the influence of a phase delay based on a signal processing time and the like.